Many of the tools already described throughout this site have social networking built right in - complete with comments, groups, and friends. However, they are usually limited to the specific tool and are probably not ideal for students. Here are couple potential tools to create social networks for your students.
Ning - This is one of the faster growing social network tools available online. Administrators have the power to make it a closed network or open it up for all to see and/or participate. Features including messaging, sharing content, forums, etc. The problem (for me at least) is that it is blocked in my district.
Google Groups - If you are already using other Google tools and wish to just hold online discussions, Google Groups is for you.
Networking
Many of the tools already described throughout this site have social networking built right in - complete with comments, groups, and friends. However, they are usually limited to the specific tool and are probably not ideal for students. Here are couple potential tools to create social networks for your students.